In Solid State Lighting (SSL), i.e. in general illumination, the relative cost contribution of the driver is expected to increase due to the decrease in Light Emitting Diode (LED) cost. Hence, in order to reduce the cost of the total system, also the driver has to become cheaper (in particular simpler and/or smaller) A closer linkage between LED and driver will still enable high performance.
When operating LEDs from a power supply, e.g. a mains voltage supply, both the electrical input waveform and the optical output waveform have to fulfill certain criteria. This holds not only for High Voltage (HV) LEDS, but the same structure may be used for with e.g. 5 low voltage LEDs in a 12V halogen replacement system, and also with e.g. 100 low voltage LEDs and mains voltage. With a Tapped Linear Driver (TLD) as e.g. described in U.S. Pat. No. 6,989,807 B2, U.S. Pat. No. 7,081,722 B1 or US 2008/0094000 A1, a method was proposed that adjusts the length of the LED string, in particular by means of shunting or bypassing these LEDs that are not to be powered from the power supply, dynamically in order to let the LED string voltage follow the wave shape/elongation of the AC supply voltage.
With this method, there is still a direct coupling between the electrical input and the optical output waveform, so influencing (reducing) the flicker or stroboscopic artifact in light will impact the input harmonics. To fulfill the various mains voltage regulations (representing a certain combination of wave shape, harmonics and power factor), a better decoupling between the electrical waveform on the input and the optical output waveform of the light is required.